Chemical reaction systems utilizing solids in contact with a gaseous or vaporized stream have long been employed. The solids may participate in the reaction as catalyst, provide heat required for an endothermic reaction, or both. Alternatively the solids may provide a heat sink in the case of an exothermic reaction. Fluidized bed reactors have substantial advantages, most notably an isothermal temperature profile. However, as residence time decreases the fluidized bed depth becomes shallower and increasingly unstable. For this reason tubular reactors employing solid-gas contact in pneumatic flow have been used and with great success, particularly in the catalytic cracking of hydrocarbons to produce gasolines where reaction residence times are between 2 and 5 seconds.
As residence times become lower, generally below 2 seconds and specifically below 1 second, the ability to separate the gaseous products from the solids is diminished because there is insufficient time to do so effectively. This occurs because the residence time requirements of separation means such as cyclones begin to represent a disproportionate fraction of the allowable reactor residence time. The problem is acute in reaction systems such as thermal cracking of hydrocarbons to produce olefins and catalytic cracking to produce gasoline using improved catalysts where the total reactor residence times between 0.2 and 1.0 seconds. In these reaction systems conventional separation devices may consume more than 35% of the allowable contact time between the two phases resulting in product degradation, coke formation, low yields and varying severity.
In non-catalytic, temperature dependent endothermic reactions, rather than separating the phases, it is possible to quench the entire product stream after the requisite reaction period. However, these solids are usually recycled and are regenerated by heating to high temperatures. A quench of the reactor effluent prior to separation would be thermally inefficient. However, it is economically viable to make a primary separation of the particulate solids before quench of the gaseous stream. The residual solids in the quenched stream may then be separated in a conventional separator inasmuch as solids gas contact is no longer a concern.
In some reaction systems, specifically catalytic reactions at low or moderate temperatures, quench of the product gas is undesireable from a process standpoint. In other cases the quench is ineffective in terminating the reaction. Thus, these reaction systems require immediate separation of the phases to remove catalyst from the gas phase. Once the catalyst has been removed, the mechanism for reaction is no longer present.
The prior art has attempted to separate the phases rapidly by use of centrifugal force or deflection means.
Nicholson U.S. Pat. No. 2,737,479 combines reaction and separation steps within a helically wound conduit containing a plurality of complete turns and having a plurality of gaseous product drawoffs on the inside surface of the conduit to separate solids from the gas phase by centrifugal force. Solids gravitate to the outer periphery of the conduit, while gases concentrate at the inner wall, and are removed at the drawoffs. Although the Nicholson reactor-separator separates the phases rapidly, it produces a series of gas product streams each at a different stage of feed conversion. This occurs because each product stream removed from the multiple product draw offs which are spaced along the conduit is exposed to the reaction conditions for a different time period in a reaction device which has inherently poor contact between solids and gases.
Ross et al U.S. Pat. No. 2,878,891 attempted to overcome this defect by appending to a standard riser reactor a modification of Nicholson's separator. Ross's separator is comprised of a curvilinear conduit making a separation through a 180.degree. to 240.degree. turn. Centrifugal force directs the heavier solids to the outside wall of the conduit allowing gases that accumulate at the inside wall to be withdrawn through a single drawoff. While the problem of product variation is decreased to some extent, other drawbacks of the Nicholson apparatus are not eliminated.
Both devices effect separation of gas from solids by changing the direction of the gas 90.degree. at the withdraw point, while allowing solids to flow linearly to the separator outlet. Because solids do not undergo a directional change at the point of separation, substantial quantities of gas flow past the withdraw point to the solids outlet. For this reason both devices require a conventional separator at the solids outlet to remove excess gas from the solid particles. Unfortunately, product gas removed in the conventional separator has remained in intimate contact with the solids, has not been quenched, and is, therefore, severely degraded.
Another drawback of these devices is the limitation on scale-up to commercial size. As conduit diameter increases the path traveled by the mixed phase stream increases proportionately so that large diameter units have separator residence times approaching those of conventional cyclones. Increasing velocity can reduce residence time, but as velocities exceed 60 to 75 ft./sec. erosion by particles impinging along the entire length of the curvilinear path becomes progressively worse. Reduction of the flow path length by decreasing the radius of curvature of the conduit also reduces residence time, but increases the angle of impact of solids against the wall thereby accelerating erosion.
Pappas U.S. Pat. No. 3,074,878 devised a low residence time separator using deflection means wherein the solid gas stream flowing in a tubular conduit impinges upon a deflector plate causing the solids, which have greater inertia, to be projected away from a laterally disposed gas withdrawal conduit located beneath said deflector plate. Again, solids do not change direction while the gas phase changes direction relative to the inlet stream by only 90.degree. resulting in inherently high entrainment of solids in the effluent gas. While baffles placed across the withdrawal conduit reduce entrainment, these baffles as well as the deflector plate are subject to very rapid erosion in severe operating conditions of high temperature and high velocity. Thus, many of the benefits of separators of the prior art are illusory because of limitations in their efficiency, operable range, and scale-up potential.